Polyethylene (PE) is synthesized via polymerizing ethylene (CH2═CH2) monomer and optionally a higher 1-olefin comonomer such as 1-butene, 1-hexene, 1-octene or 1-decene. Because PE is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the synthesis methods, PE can be generally classified in to several types such as LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
It is known that the polymerisation of olefins e.g. ethylene, especially by a gas phase polymerisation process, involves the polymerisation of olefin monomer with the aid of catalyst and optionally, if required depending on the used catalyst, a co-catalyst. Suitable catalysts for use in the production of polyolefins, and in particular for the preparation of polyethylene, comprise chromium-type catalysts, Ziegler-Natta catalysts and metallocene catalysts.
It is well known that the polymerisation reaction is quite sensitive to the quantity of catalyst utilized. It is important to control catalyst flow to a reactor since unexpected or uncontrolled catalyst injection in a reactor could lead to runaway reactions. However, one of the major problems in the injection of catalyst slurry to a reactor in prior art methods is that it is difficult to control the amount of catalyst and the flow rate of the catalyst injected.
According to prior art catalyst supply systems, catalyst may be provided to a polymerization reaction either in concentrated form, e.g. directly from a mud pot, or in diluted form.
Direct feeding of catalyst slurry from a storage vessel to a reactor has the disadvantage that the feeding rate of the catalyst to the reactor cannot be adequately controlled. A Iso, in cases involving direct supply of a (concentrated) catalyst to a reactor, the catalysts can completely be flushed in the reactor, when a problem occurs during the preparation of the catalysts. Such uncontrolled catalyst supply may induce runaway reactions in the reactor.
Moreover, in the case catalyst in oil suspension is provided directly to a reactor, the used pumps, generally progressive cavity pumps, are not able to dose the catalyst flow and the amount of catalyst injected in the reactor. Furthermore, such systems require the switch over of the catalyst injection system, every time a new batch of catalyst needs to be connected to the reactor for supply thereto. Therefore, such injection systems do not provide an optimal and reliable control of the catalyst flow rate.
Several systems have been disclosed which involve the preparation and the supply of diluted catalyst slurry to a polymerization reaction. In general, for preparing catalyst slurry, a mixture of dry solid particulate catalyst and diluent are apportioned in a catalyst storage vessel for thorough mixing. Then such catalyst slurry is typically transferred directly to a polymerization reaction vessel for contact with the monomer reactants, generally under high pressure conditions.
GB 838,395 relates to a process and apparatus for producing a slurry of a solid catalyst in hydrocarbon diluent for use in a chemical reaction. The process comprises preparing concentrated catalyst slurry in a hydrocarbon diluent and admixing said concentrated slurry with additional diluent and introducing said admixture to a reaction zone. According to the process, the specific inductive capacity of the slurry is continuously determined prior to the introduction of same to said reaction zone, the inductive capacity of the slurry being dependent upon the concentration of catalyst in the slurry.
U.S. Pat. No. 3,726,845 describes a system wherein catalyst slurry is prepared in a vessel after which it is pumped to the polymerization reactor by means of a conduit provided with a pump. The catalyst slurry formed in the vessel and diluent are alternately fed through said conduit to the polymerization reactor by flowing catalyst for a selected number of seconds and then diluent for a selected number of seconds through the conduit to the polymerization reactor.
WO 2004/0264455 describes a catalyst slurry feeding system wherein diluted catalyst slurry is formed in a mixing tank and transferred to a storage tank, wherein it is maintained in diluted form before being supplied to a polymerization reactor. Catalyst slurry is supplied from the mixing to the storage tank by means of a conduit provided with a valve. The mixing tank can be at a higher elevation than the storage tank, so that the catalyst slurry flows from the mixing tank to the storage tank at least partially due to gravity, thereby avoiding the necessity of a pump between the mixing tank and the storage tank. Alternatively, the catalyst slurry can be moved between the tanks without a pump or a difference in elevation by maintaining a pressure differential between the mixing tank and the storage tank.
U.S. Pat. No. 5,098,667 discloses a catalyst supply system involving the transfer of concentrated catalyst from a mud pot to a dilution vessel through a conduit which includes a valve system for regulating the transfer. Diluted catalyst slurry is continuously supplied to the polymerization reactor by means of a conduit. In the described method the flow rate of the diluted slurry is manipulated so as to provide a desired flow rate of solid particles contained in the diluted slurry. Continuous catalyst flow is maintained at a desired rate in response to a computed value of the mass flow rate of the solid catalyst particles contained in the dilute slurry. The computed mass flow rate of catalyst particles is based upon “on line” measurements of density and flow rate of the dilute catalyst slurry stream flowing to the reactor, and on predetermined densities of the solid catalyst particles and the liquid diluent constituting the slurry.
However, although the above-described methods for preparing diluted catalyst provide an improvement on the control of catalyst flow, they have the disadvantage that the catalyst flow rate can not be reliably adjusted in function of the reaction conditions in the polymerisation reactor.
Another problem associated with available systems for preparing diluted catalyst is that these systems are relatively voluminous and cumbersome, and sometimes involve several tanks for storing diluted catalyst slurry. In addition, the use of large vessels and large amounts of diluent for preparing the diluted catalyst slurry implies several considerable disadvantages. The use of large volumes of diluted catalyst implies the use of large volumes of diluent (i.e. isobutane). Isobutane however, is an explosive chemical and may pose security problems.
Furthermore, when switching of catalyst type in a polymerization process, large amounts of catalyst material may remain unused and may need to be disposed off, which is not only very expensive but also implicates severe environmental regulations. In addition, the costs for cleaning the voluminous catalyst preparation systems after removal of the catalyst are high.
Therefore, there remains a need in the art for providing an improved method for controlling catalyst feeding to a polymerization reactor. More in particular, there remains a great need in the art for a system that continuously and reliably delivers diluted catalyst slurry to a loop reactor.
Furthermore, metallocene and Ziegler-Natta catalysts are usually employed with a co-catalyst for olefin polymerization, which can significantly enhance the polymerization efficiencies to beyond a million units of polymer per unit of catalyst. A number of techniques for the introduction of the co-catalyst to a polymerization reactor has been proposed. For instance some techniques consist of introducing the co-catalyst directly into the polymerization reactor. However, such technique does not allow bringing the co-catalyst into contact with the catalyst before entering the reactor, although such pre-contact is particularly desirable in order to provide effective catalyst-co-catalyst mixtures. Another technique consists of contacting the catalyst and co-catalyst before their introduction into the polymerization medium. In this latter case, however, having regard to the fact that the catalyst systems employed usually have maximum activity at the commencement of polymerization, it may be difficult to avoid reaction runaways liable to involve the formation of hot spots and of agglomerates of molten polymer.
In view hereof, it can be concluded that there remains also a need in the art for providing an improved method for controlling catalyst feeding, in pre-contact with a co-catalyst, to a polymerization reactor.
It is therefore a general object of this invention to provide an improved method and apparatus for optimising catalyst introduction in a polymerisation reactor. It is a particular object of the present invention to optimise the supply of a catalyst, commercially provided in an oil suspension or in a hydrocarbon solution, to a polymerisation reactor wherein polyethylene is prepared. More in particular, the present invention also aims to provide an apparatus and method enabling to effectively control the flow rate of a catalyst to a polymerisation reactor wherein polyethylene is prepared.
It is a further object of the present invention to provide an apparatus and method for controlling catalyst feeding, being in pre-contact with a co-catalyst, to a polymerisation reactor, wherein polyethylene is prepared.
Furthermore, the present invention aims to provide a method and an apparatus for improved control of the polymerization reaction of ethylene in a reactor.